The photochemistry of 4-chlorophenol has been studied in aqueous solution as a function of pH, 4-chiorophenol concentration and oxygen concentration. Solutions of 4-chlorophenol in the concentration range around 5 x 10-4 mol dm-3 have been photolysed by being subjected to a series of microsecond white light flashes and the result of the photolysis analysed by HPLC. In aerated solution, para-benzoquinone is the main photoproduct together with smaller amounts of para-hydroquinone and 2-hydroxy-1,4-benzoquinone. The latter is found to be the dominant photoproduct at pH values above the pKa of 4-chlorophenol (9.4). A quantum yield for reaction of the 4-chlorophenol in neutral solution of 0.44 ± 0.1 has been determined by actinometry. The photolyses of 4-bromophenol and 4-iodophenol in neutral aqueous solution yield the same photoproducts as for 4-chiorophenol but with reduced quantum yields of 0.08 and 0.022 respectively. In oxygen-saturated solution the rate at which 4-chlorophenol is photolysed is virtually identical to the rate in aerated solution. However it is found that the photoproduct distribution is further dominated by para-benzoquinone with para-hydroquinone only being detected after a significant period of photolysis. In degassed solution the product distribution is very different. Here a larger range of photoproducts is found but para-benzoquinone is not amongst them, indicating that molecular oxygen is necessary for its formation. LC/MS has been used to help identify some of the photoproducts which include para-hydroquinone, 2,2' - and 4,4'-dihydroxybiphenyl and 2,4',5-trihydroxybiphenyl. The use of nanosecond laser flash photolysis has revealed the formation of two transient species in aerated or oxygen-saturated solution. These have been identified as 4-oxocyclohexa-2,5-dienylidene and para-benzoquinone-O-oxide. The latter is derived from the former by reaction with oxygen with a rate constant of 1.9 x 109 dm3 mol 1 S4. 4-oxocyclohexa-2,5-dienylidene, which is a triplet carbene, is derived from all four 4-halogenophenols by loss of the respective hydrogen halide. The hydrogen halide elimination appears to be partially reversible in that the yield of transients is sensitive to the halide concentration in the solution. Both transients are sensitive to the halogenophenol concentration and to the presence of protons. At high pH or in degassed solution the carbene is still formed but in greatly reduced yields and long-lived radical species are also detected. There is a strong heavy atom effect on the fluorescence properties of the 4-halogenophenols in ethanol and water and the 4-halogenoanisoles in ethanol. Fluorescence yields of the order of 0.1 and lifetimes of 2.0 ns for the fluoro compounds are reduced by a factor of approximately 5 for the chloro compounds and by factors ~t 100 for the bromo and iodo derivatives. These observations, taken together with the effect of the halogen on the transient yields and the reaction quantum yields indicate that the reactive halogenophenol excited state is the first excited singlet state.